The invention relates to a process for coating hollow bodies, which are open at one end, such as metal cans provided with a bottom, with lacquer or the like, in which the individual hollow bodies are washed, coated on the inside and on the outside and dried and then optionally printed and dried once again and moreover flanged at the open end.
The increasingly tightened requirements of environmental protection lead to considerations of how processes, such as the electrophoretic lacquering process, which is also referred to as electrocoating (EC), can be introduced as a fully automatic lacquering process in the can manufacturing industry. The processes of lacquering the sides of the three-part cans or of coating a weld electrophoretically by immersion in an electrophoretic immersion bath are well known (U.S. Pat. No. 3,694,336, German Offenlegungsschrift No. 2,116,715). The can bodies are easily handled here, because they do not yet have a bottom and the bath liquid can enter without difficulties for the coating process and can leave once again equally well without difficulties after the coating process.
Hollow bodies, which are closed on one side, such as cans provided with a bottom, cannot easily be coated electrophoretically, because it is necessary that the air in the hollow body escapes completely so that a uniform coating will be produced. The engineering industry therefore developed special methods, in which the process is carried out in steps, that is, lacquering is carried out in individual, consecutive steps, for example, on the inside to start with. The contructions, known for this purpose, have some common features. For instance, the cans are held at the bottom for lacquering the inside and the necessary electrical contacts are produced at the same time. A counterelectrode is installed in the open end of the can and must be at a small distance of 0.25 to 5 mm from the inside wall of the can. The shape of the electrode therefore has to be adjusted very accurately to that of the can. Because of the complicated construction of the appropriate equipment, the cans must be coated individually and consecutively, so that only short coating times of 10 to 500 msec are available, if a high throughput of cans is to be achieved. In closed systems with, for example, a vertical arrangement (European Pat. Nos. 50,045 and 19,669, British Patent No. 1,117,831, U.S. Pat. No. 3,922,213 and German Offenlegungsschrift No. 2,929,570), the liquid must be pumped at high speeds in order to be able to carry out (coating with) the EC liquid and rinsing with water alternately in short time spans and to remove the gases formed during the EC coating process (oxygen or hydrogen, depending on the polarity). In open systems, the cans, which are arranged approximately horizontally, must be turned in order to achieve a uniform coating (German Patent No. 2,633,179 and U.S. Pat. No. 4,107,016). There is great danger of contamination when blowing out the cans.
The disadvantage of all of these known constructions lies therein that the cans must be coated individually and consecutively with great mechanical effort. The large space required for the equipment makes economic mass production almost impossible. The internal electrodes can be inserted so as to fit accurately only in cans with smooth, straight walls; that is, can shapes, which deviate from the cylindrical, cause great difficulties. Because of the small distances between the internal electrode and the can wall, there is a risk of short circuits as well of disruptive electrical discharges in zones of very high current density. Lacquers must therefore be used, which make trouble-free coating processes possible at suitable voltages in the short times available.